Dielectric resonator antennas (DRAs) are known as miniaturized antennas of ceramics or another dielectric medium for microwave frequencies. A dielectric resonator whose dielectric medium, which has a relative permittivity of .di-elect cons..sub.r &gt;&gt;1, is surrounded by air, has a discrete spectrum of eigenfrequencies and eigenmodes due to the electromagnetic limiting conditions on the boundary surfaces of the dielectric medium. These conditions are defined by the special solution of the electromagnetic equations for the dielectric medium with the given limiting conditions on the boundary surfaces. Contrary to a resonator, which has a very high quality when radiation losses are avoided, the radiation of power is the main item in a resonator antenna. Since no conducting structures are used as a radiating element, the skin effect cannot be detrimental. Therefore, such antennas have low-ohmic losses at high frequencies. When materials are used that have a high relative permittivity, a compact, miniaturized structure may be achieved since the dimensions may be reduced for a preselected eigenfrequency (transmission and reception frequency) by increasing .di-elect cons..sub.r. The dimensions of a DRA of a given frequency are substantially inversely proportional to .di-elect cons..sub.r. An increase of .di-elect cons..sub.r by a factor of .alpha. thus causes a reduction of all the dimensions by the factor .alpha. and thus of the volume by a factor of .alpha..sup.3/2, while the resonant frequency is kept the same. Furthermore, a material for a DRA is to be suitable for use at high frequencies, have small dielectric losses and temperature stability. This strongly limits the materials that can be used. Suitable materials have .di-elect cons..sub.r values of typically a maximum of 120. Besides this limitation of the possibility of miniaturization, the radiation properties of a DRA degrade with a rising .di-elect cons..sub.r.
Such a DR antenna 1 in the basic form considered by way of example is represented in FIG. 1. Not only the form of a cuboid, but also other forms are possible such as, for example, cylindrical or spherical geometries. Dielectric resonator antennas are resonant modules that work only in a narrow band around one of their resonant frequencies (eigenfrequencies). The problem of the miniaturization of an antenna is equivalent to the fact of lowering the operating frequency with given antenna dimensions. Therefore, the lowest resonance (TE.sup.z.sub.111) mode is used. This mode has planes of symmetry in its electromagnetic fields, of which one plane of symmetry of the electric field is referenced plane of symmetry 2. When the antenna is halved in the plane of symmetry 2 and an electrically conducting surface 3 is deposited (for example, a metal coating), the resonant frequency continues to be equal to the resonant frequency of an antenna with the original dimensions. In this manner, a structure is obtained in which the same mode is formed with the same frequency. This is represented in FIG. 2. A further miniaturization can be achieved with this antenna by means of a dielectric medium that has a high relative permittivity .di-elect cons..sub.r. Preferably, a material that has low dielectric losses is selected.
Such a dielectric resonator antenna is described in the article "Dielectric Resonator Antennas--A review and general design relations for resonant frequency and bandwidth", Rajesh K. Mongia and Prakash Barthia, Intern. Journal of Microwave and Millimeter-Wave Computer-aided Engineering, vol. 4, no. 3, 1994, pp. 230-247. The article gives an overview of the modes and the radiation characteristics for various shapes, such as cylindrical, spherical and rectangular DRAs. For different shapes, the possible modes and planes of symmetry are shown (see FIGS. 4, 5, 6 and p. 240, left column, lines 1-21). Particularly a cuboidal dielectric resonator antenna is described in the FIG. 9 and the associated description. By means of a metal surface in the x-z plane, with y=0, or in the y-z plane, with x=0, the original structure may be halved, without modifying the field configuration or other resonance characteristics for the TE.sup.z.sub.111 -mode (p. 244, right column, lines 1-7). The DRA is excited via a microwave lead in that it is inserted into the stray field in the neighborhood of a microwave line (for example, a microstrip line or the end of a coaxial line).
The possibility of reducing the volume is limited to the use of the two planes of symmetry arranged at right angles to each other as outside surfaces. In this manner, the volume of a DRA may be reduced only by the factor of 4 with the same frequency.